Some organisations, such as hospitals, are compelled by regulation to dispose of their waste material by incineration. In order to reduce the cost, incurred due to the fuel consumption of the furnace, of incinerating the waste material, attempts have been made to recover and utilize the waste heat produced during the incineration process. The most cost-effective methods of recovery proposed thus far, involve systems using a waste heat recovery boiler, capable of raising steam for, example a central heating system, connected to a pyrolitic incinerator. Such systems must, however, satisfy the strict regulations concering air pollution. The systems must also be regularly loaded with waste material and regularly de-ashed, in order to prevent build-up, if the system is to operate efficiently. This necessitates constant supervision and maintenance, and although mechanisms for the automatic de-ashing of furnaces have been proposed, the mechanisms presently in use or disclosed in GB No. 1275304A (FABRY), which attempt to avoid any interruption of the incineration process, can be considerably complex.